Travis CI: make lint-diff mandatory

[cclauss]

Improved automated testing

• Run 450+ Python doctests on all Travis CI runs
• Run tests on Python 3.9 release candidate 1 in allow_failures mode.
• Drop an obsolete Python 3.8 run that is surpassed by normal test runs.

Technical

Testing

Screenshot

Stakeholders

[cdrini]

This is different; this runs the more aggressive flake8 against the diff, no? We still want that (unless our code has become compatible with the stricter flake8 config)

[cclauss]

A failure of make lint-diff does not currently fail the Travis CI test run.
There are two issues with the way that we are running make lint-diff:

1. The Travis build passes even when make lint-diff fails.
2. Flake8 is choking when handed binary data like .mrc files.

I believe that these two issues are easy to fix but we would start failing the build when contributors fix typos. I believe that we would be better served by blackening our code before we make make lint-diff mandatory. If not, we will frustrate our contributors with rework. Manual rework may cause bugs. If we do not blacken first then we will spend a lot of code review time on educating contributors on code formatting rules related to parts of the lines that they do not wish to change.

[cdrini]

Yeah, let's enable this + make it mandatory. I don't think it'll be that annoying, since it'll only complain about the lines people are editing.

To fix 2, would adding a --filename=*.py to scripts/flake8-diff.sh do the trick?

[cdrini]

I think we should consider blackening our code in the future, but not sure this is the best time considering how many things we have on our plate 😅

[cdrini]

Awesome! Let's give that a shot :)